Commonly-owned U.S. Pat. Nos. 5,677,390, 5,686,538, 5,767,203, 5,852,131, 5,852,132, 5,916,978, 6,133,381, 6,207,765, 6,232,393, 6,232,408, 6,235,831, 6,235,844, 6,326,439, and 6,521,714 describe what is believed to be the best known prior process technology for producing brominated styrenic polymers such as brominated polystyrene having the best known properties of any previously-known brominated styrenic polymer. In this connection, the terms “brominated styrenic polymer” and “brominated polystyrene” as used in the specification and in the claims hereof refer to a brominated polymer produced by bromination of a pre-existing styrenic polymer such as polystyrene or a copolymer of styrene and at least one other vinyl aromatic monomer, as distinguished from an oligomer or polymer produced by oligomerization or polymerization of one or more brominated styrenic monomers, the properties of the latter oligomers or polymers typically being considerably different from brominated polystyrene in a number of respects.
In producing brominated anionic styrenic polymers by bromination of anionic styrenic polymers, bromobenzene impurities tend to be formed in greater than desired quantities. Among these impurities are species in which benzene rings can be substituted by 2 to 6 bromine atoms. Because of their greater volatility at elevated temperatures encountered in molding, the species containing 2, 3, or 4 bromine atoms as ring substituents are more undesirable than those containing 5 or 6 bromine atoms on the ring. In the case of bromination of anionic polystyrene using aluminum halide catalysts in which the halide atoms are bromine or chlorine or both, the species containing 2, 3, or 4 bromine atoms as ring substituents as determined by NMR are, respectively, 1,4-dibromobenzene, 1,2,4-tribromobenzene, and 1,2,4,5-tetrabromobenzene. These volatile species have a strong odor and are considered to be skin and lung irritants. Also, the formation of these species in the process results from cleavage of aromatic rings from the polymer chain. This in turn introduces irregularities in the polymer chain and/or addition of bromine atoms to the polymer chain. Such addition results in reducing the thermal stability of the brominated styrenic polymer. Thus, it would be highly advantageous if a way could be found for reducing the formation of bromobenzene impurities, especially the more volatile dibromo, tribromo and/or tetrabromo species, during the actual preparation of brominated anionic styrenic polymers rather than relying upon purification steps during product workup or purification.